Is the problem just knowing what the USB path/drive letter is? If so you can use the /home alias to get the folder that Opus is running out of and the use relative paths from that to find UltraEdit.
I seem to remember that U3 has its own launcher, though, so many you need to run that with arguments telling it to launch UltraEdit... If so I'm not sure how that works as when I've tried out U3 I've only launched things via its taskbar icon. Maybe someon else will know more, though.
That does not work because you don't have a regular executable with U3.
Manually launching is only possible by the U3-Menu; no chance to start the launcher with some arguments manually